1. Field of the Invention
The present invention relates to a method and apparatus for the electrical nondestructive testing of metals.
2. Description of the Prior Art
The electrical resistivity of metals and alloys has been extensively studied. Resistivity measurements have been widely used as a research tool for characterizing phase transitions. In addition, the present inventors have discovered that electrical resistivity changes can be a source of erratic weld penetration in electrical spot welding; see R. L. Cohen and K. W. West, Tooling and Production, page 94 (February 1981). Thus, production line control applications can benefit from a quick and easy technique to determine the resistivity of a metal. Similarly, incoming material certification and stockroom sorting procedures would benefit from a quick and easy means of determining resistivity. It is, however, notoriously difficult to make good electrical contact to commercial steels: the surfaces are frequently rough, covered with corrosion spots, oiled, and sometimes passivated, painted, or plated. This has made direct measurement of resistivity by contact techniques impractical. The traditional approach to measuring resistivity is to cut a test sample of small, well-controlled cross section and make a precise measurement of the voltage drop along the sample with a precisely known current. However, it is apparent that this destructive technique is impractical for real-time measurement along a production line, and inconvenient for other applications, including acceptance testing or sorting.
Instruments using eddy current techniques have been widely used for determining the electrical resistivity of aluminum and nonmagnetic stainless steels. These measurements have been used for flaw and microcrack detection and for alloy certification; that is, as a simple way to determine which alloy a particular sheet is made of. The simplicity of the eddy current technique, due to the fact that no electrical contacts are made to the sample, no specially shaped samples need to be cut, with access being required only to one side, and the fact that the instrumentation can be portable and battery operated, contributes to the broad application of this approach. Eddy current measurements have also been extended to study semiconductor materials. However, the eddy current measurement requires significant effort in calibration to account for variations in material geometry which influence the eddy currents measured. In addition, for steels and other alloys of significant magnetic permeability (e.g., .mu.&gt;1.05), eddy current determinations need substantial corrections to account for permeability variations, and are thus of limited utility. Thus, electrical resistivity measurements have not been widely used in ferrous metallurgy.
However, as noted above, there are many possible applications of a resistivity measurement, especially in view of the locally inhomogeneous composition of many steel products as a result of segregation of the alloying ingredients during solidification of the ingot. It is desirable that the technique be easily applied, with a minimum of sample preparation and a minimum of instrument calibration. It is especially desirable that it be fast, of high enough accuracy to allow distinguishing alloy compositions, and useful for a wide range of sample sizes.